In recent years, a technological explosion has occurred in the ink jet printing art which has substantially advanced the quality of resultant printed images. However, such systems have remained generally bulky and immobile, thereby reducing the range of tasks open to them. Furthermore, their physical construction is such that they do not allow their operators to be conveniently stationed with regard to ink and solvent supplies and the supervising control apparatus so that they may respond immediately to problems or changes in the printing process. Efforts have been made to introduce systems which are physically more flexible, such as the large, partially mobile system disclosed in U.S. Pat. No. 4,283,731 and the multiple component system disclosed in U.S. Pat. No. 3,913,719, but a compact, fully self-contained and mobile system has not yet been introduced.
The flexibility and efficiency of current systems has been further enhanced by the introduction of microprocessors which are used to control and change type sizes and font styles while printing at extremely high speeds. Microprocessors may also be employed in systems used to print code information on moving packages by responding to a signal produced by a package sensor to cause the code information to be properly positioned on the package such as illustrated in U.S. Pat. No. 4,283,731. In addition to the control features mentioned above, microprocessors may also regulate such technical aspects of the ink jet printing head, as the timing of signal pulses sent to each of several solenoid valve units instructing them when and how long to open so as to simultaneously deliver the proper amount of ink to each of several nozzles contained in the head for deposit on a printing surface. This type of system is shown in U.S. Pat. No. 4,215,350 where a number of solenoid valves are controlled by a pattern generator which regulates the height of printed characters by sending pulses to one of two groups of solenoid valves causing the appropriate valves in that group to open simultaneously. The problem which arises is that the ink in such systems must travel different distances to the proper nozzle. Therefore, it is difficult to maintain uniform pressure at each nozzle head, and, when the pressure is not uniform, the ink droplets deposited by each nozzle may vary in size, reducing the quality of printing obtainable from the system. An attempt to improve uniformity is illustrated by U.S. Pat. No. 4,284,993 in which an ink pressure equalization pipe is used but this patent does not address the problem of obtaining uniformity in a system employing solenoid valves for forming ink jet droplets from stationary discharge orifices.
The efficiency and economy of these high speed systems is further adversely effected by difficulties encountered in manufacturing adequate nozzle heads. The small size of nozzles and the nozzle orifices make them difficult to manufacture reproducibly and also makes the identification of sources of variable performance difficult to diagnose. One effort to overcome these problems is disclosed in U.S. Pat. No. 4,282,533 in which a unitary metallic nozzle member is used to provide strength, fluid flow properties and to control pressure drop across the orifices. However, the high cost of manufacturing nozzles to close tolerances remains, as do the uniform pressure problems mentioned above.
Other known features in ink jet printing systems include multiple filter systems for ensuring a supply of clean ink and solvent (U.S. Pat. No. 4,153,902); automatic, three-way, valve-controlled devices for cleaning ink jet heads with solvent (U.S. Pat. No. 4,296,418); and the addition of an ink reservoir to a nozzle for improving system start-up performance (U.S. Pat. No. 4,187,512). However, none of these references discloses a fully, mobile, self-contained ink jet printing system which overcomes the problems noted above.